1. Technical Field of the Invention
The present invention relates to spread spectrum communications systems and, in particular, to the handling of the various channels transmitted on the downlink from a base station operating in a spread spectrum communications system.
2. Description of Related Art
The cellular telephone industry has made phenomenal strides in commercial operations throughout the world. Growth in major metropolitan areas has far exceeded expectations and is outstripping system capacity. If this trend continues, the effects of rapid growth will soon reach even the smallest markets. The predominant problem with respect to continued growth is that the customer base is expanding while the amount of electromagnetic spectrum allocated to cellular service providers for use in carrying radio frequency communications remains limited. Innovative solutions are required to meet these increasing capacity needs in the limited available spectrum as well as to maintain high quality service and avoid rising prices.
Currently, channel access is primarily achieved using Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) methods. In frequency division multiple access systems, a physical communication channel comprises a single radio frequency band into which the transmission power of a signal is concentrated. In time division multiple access systems, a physical communications channel comprises a time slot in a periodic train of time intervals over the same radio frequency. Although satisfactory performance is being obtained from FDMA and TDMA communications systems, channel congestion due to increasing customer demand commonly occurs. Accordingly, alternate channel access methods are now being proposed, considered and implemented.
Spread spectrum comprises a communications technique that is finding commercial application as a new channel access method in wireless communications. Spread spectrum systems have been around since the days of World War II. Early applications were predominantly military oriented (relating to smart jamming and radar). However, there is an increasing interest today in using spread spectrum systems in communications applications, including digital cellular radio, land mobile radio, and indoor/outdoor personal communication networks.
Spread spectrum operates quite differently from conventional TDMA and FDMA communications systems. In a direct sequence code division multiple access (DS-CDMA) spread spectrum transmitter, for example, a digital symbol stream for a given dedicated or common channel at a basic symbol rate is spread to a chip rate. This spreading operation involves applying a channel unique spreading code (sometimes referred to as a signature sequence) to the symbol stream that increases its rate (bandwidth) while adding redundancy. Typically, the digital symbol stream is multiplied by the unique digital code during spreading. The intermediate signal comprising the resulting data sequences (chips) is then added to other similarly processed (i.e., spread) intermediate signals relating to other channels. A base station unique scrambling code (often referred to as the "long code" since it is in most cases longer than the spreading code) is then applied to the summed intermediate signals to generate an output signal for multi-channel transmission over a communications medium. The dedicated/common channel related intermediate signals advantageously then share one transmission communications frequency, with the multiple signals appearing to be located on top of each other in both the frequency domain and the time domain. Because the applied spreading codes are channel unique, however, each intermediate signal transmitted over the shared communications frequency is similarly unique, and through the application of proper processing techniques at the receiver may be distinguished from others.
In the DS-CDMA spread spectrum mobile station (receiver), the received signals are recovered by applying (i.e., multiplying, or matching) the appropriate scrambling and spreading codes to despread, or remove the coding from the desired transmitted signal and return to the basic symbol rate. Where the spreading code is applied to other transmitted and received intermediate signals, however, only noise is produced. The despreading operation thus effectively comprises a correlation process comparing the received signal with the appropriate digital code to recover the desired information from the channel.
In one known prior art transmitter implementation, the spreading process used by the base station for the common channel type broadcast control channel (BCCH) is a little different than that implemented with respect to the other dedicated/common channels. The pilot codes for the base station, as well as the long (scrambling) code group codes that provide information indicative of which long code is being used by the base station, are embedded within the broadcast control channel information. These pilot codes and long code group codes are transmitted periodically. The time intervals during which these codes are sent last for one symbol, and are referred to as "long code masked symbols". At each instance of long code masked symbol transmission, the pilot codes and the long code group codes are sent instead of sending the broadcast control channel. This is accomplished by turning off the broadcast control channel information stream, and instead transmitting the pilot code modulated by a known symbol (such as "+1"). At the same time, the long code group code is similarly transmitted, again modulated by a known symbol (such as "+1") As these codes are transmitted simultaneously, it is preferable that the pilot codes and the long code group codes be orthogonal to each other. Furthermore, for only these code transmissions, the base station unique digital code (the "long code") is removed. This is accomplished, for example, by multiplying the pilot codes and the long code group codes by the complex conjugate of the long code.
From an implementation point of view, the scheme described above for implementing the transmission of the long code masked symbols requires complicated and specific code channel signal processing (both on the hardware side and software side) for the broadcast control channel that differs from that required for the other dedicated/common channels. It would be more economical and efficient to have a unified processing resource (hardware and/or software) for all channels on the downlink. It is also recognized that during the transmission of the long code masked symbols no broadcast control channel symbols are sent. This adversely affects the bit rate of the broadcast control channel. Preferably, the unified processing resource should improve the information transmission rate of the broadcast control channel.